Hydrogenation catalysts, which enhance reactions between hydrogen and other compounds, are a topic of significant interest. In particular, catalysts for CO/CO2 hydrogenations into higher hydrocarbons are of specific interest. These products are widely used as solvents, intermediates, fuel additives, and neat fuels, and producing these products selectively requires catalysts with specific properties. Typically, major byproducts from CO/CO2 hydrogenations are single carbon compounds, so a major challenge is the development of catalysts with higher selectivity towards the higher chain hydrocarbons. There is particular emphasis on high selectivity catalysts for CO/CO2 hydrogenations acting in environments of CO, CO2, and H2, such as syngas, which require relatively high stability in the presence of a reducing environment. Generally a variety of catalysts, particularly the Group 6 through 11 metals, have been employed in CO/CO2 hydrogenations, but in many cases these catalysts generate broad, complex mixtures of hydrocarbons, oxygenated hydrocarbons, and carbon dioxide. Thus, there is a need for CO/CO2 hydrogenation catalysts that selectively generate long-chain hydrocarbons products.
Certain metals such as Rh. Co, and Fe have been extensively studied because of their generally high hydrogenation activities. The activity and selectivity of the active metal catalysts can be increased by various factors, such as the presence of promoters, the choice of support, the synthesis method, the use of specific precursors, and other factors. Generally, optimum long-chain hydrocarbon formation requires a balance among the rates of CO dissociation, hydrogenation, and methyl group insertion. For example, promoters such as rare earth metals, alkali metals, and other transition metals play an important role in these elementary steps. Typically the promoters activate the oxygen atom of an absorbed CO molecule and weaken the C—O bond, leading to CO dissociation followed by a hydrogenation step to form CHx species. The mechanism for C—C bond formation leading to the formation of long-chain hydrocarbons also requires the atomic proximity of another activated CHx species. Subsequent insertion steps and hydrogenation of the initial C2 intermediate leads to higher chain hydrocarbon synthesis. Catalytic metals have also been supported on certain crystalline oxides such as pyrochlores in an effort to promote selectivity in CO/CO2 hydrogenations. CO/CO2 hydrogenation catalysts have involved catalytic metals such as Co, Cu, and Rh supported by various structures such as La2Zr2O7, La2FeO3, La2O3, TiO2, SiO2, and Al2O3. See Kieffer et al., “Hydrogenation of CO and CO2 toward methanol, alcohols and hydrocarbons on promoted copper-rare earth oxides catalysts,” Catalysis Today 36 (1997); and see Chu et al., “Conversion of syngas to C1-C6 alcohol mixtures on promoted CuLa2Zr2O7 catalysts.” Applied Catalysis A: General 121 (1995); and see Fujiwara et al., “Hydrogenation of carbon dioxide over copper-pyrochlore/zeolite composite catalysts,” Catalysis Today 29 (1996). The efforts are generally aimed toward adjustment of the CO dissociation and insertion abilities of the Co, Cu, or Rh through varying promoter and support compositions. Variations in selectivities are typically attributed to the specific properties of the support, the promoter, the morphology of the metal, and the impact of the support on the reducibility of the metal.
Provided here is a method of hydrogenation utilizing a reactant gas mixture comprising a carbon oxide and a hydrogen agent, and a hydrogenation catalyst comprising a mixed-metal oxide with a metal site supported by and/or incorporated into the lattice. In an embodiment, the metal site is a deposited metal and the mixed-metal oxide supports the metal site. The metal site comprises a transition metal, an alkali metal, an alkaline earth metal, or mixtures thereof, and the conducting oxide comprises a pyrochlore, a brownmillerite, or mixtures thereof, typically doped at an A-site or B-site of the conducting oxide crystal structure. Contact between the carbon oxide, hydrogen agent, and hydrogenation catalyst under appropriate conditions of temperature, pressure and gas flow rate generate a reaction and products comprising paraffins, olefins, or combinations thereof, where the paraffins, olefins, or combinations comprise carbon from the carbon oxide and some portion of the hydrogen agent. The carbon oxide may be CO, CO2, or mixtures thereof and the hydrogen agent may be H2.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.